Versatile Polarization Generation with an Aluminum Plasmonic

Subscriber access provided by NEW YORK UNIV

Communication

Versatile polarization generation with aluminum plasmonic metasurface Pin Chieh Wu, Wei-Yi Tsai, Wei Ting Chen, Yao-Wei Huang, Ting-Yu Chen, JiaWern Chen, Chun Yen Liao, Cheng Hung Chu, Greg Sun, and Din Ping Tsai Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04446 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Versatile polarization generation with aluminum plasmonic metasurface Pin Chieh Wu,1,2 Wei-Yi Tsai,2 Wei Ting Chen,2 Yao-Wei Huang,1 Ting-Yu Chen,2 Jia-Wern Chen,2 Chun Yen Liao,2 Cheng Hung Chu,1 Greg Sun,3 and Din Ping Tsai,1,2,4,*

1

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan

2

Department of Physics, National Taiwan University, Taipei 10617, Taiwan

3

Department of Engineering, University of Massachusetts Boston, Boston, Massachusetts 02125, United

States 4

College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan

*[email protected]

ABSTRACT

All forms of light manipulation rely on light-matter interaction, the primary mechanism of which is the modulation of its electromagnetic fields by the localized electromagnetic fields of atoms. One of the important factors that influence the strength of interaction is the polarization of the electromagnetic field. Generation and manipulation of light polarization have been traditionally accomplished with bulky optical components such as waveplates, polarizers, and polarization beam splitters that are optically thick. Miniaturization of these devices is highly desirable for the development of a new class of compact, flat, and broadband optical components that can be integrated together on a single photonics chip. Here we demonstrate, for the first time, a reflective metasurface 1

ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 38

polarization generator (MPG) capable of producing light beams of any polarizations all from a linearly polarized light source with a single optically thin chip. Six polarization light beams are achieved simultaneously including four linear polarizations along different directions and two circular polarizations, all conveniently separated into different reflection angles. With the Pancharatnam-Berry phase-modulation method, the MPG sample was fabricated with aluminum as the plasmonic metal instead of the conventional gold or silver, which allowed for its broadband operation covering the entire visible spectrum. The versatility and compactness of the MPG capable of transforming any incident wave into light beams of arbitrary polarizations over a broad spectral range are an important step forward in achieving a complete set of flat optics for integrated photonics with far-reaching applications. KEYWORDS: Metasurfaces,

Aluminum plasmonics, Polarization generator, Nanoantennas,

background-free, Pancharatnam-Berry phase modulation Introduction. Since its discovery in the early 19th century, polarization as one of the key properties of an electromagnetic wave has been used in a wide variety of applications from glare-reducing glasses, liquid-crystal displays1, to optical signal processing2, all of which have had a profound impact in our modern day life. The simplest way of generating a polarized light is to use a polarizer that functions as an optical transmissive filter to allow light of a specific polarization pass but blocking waves of other polarizations. Such a device generates linearly polarized light at the expense of brightness as wave intensity contained in the perpendicular polarization is simply lost3. Ideally, one 2


Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

would prefer a polarization generator capable of preserving as much as possible the power intensity contained in the incident waves of all polarizations and converting them into a beam or a set of beams of well-defined polarizations. Such an optical component is obviously desirable when signal strength is a matter of concern for applications involving manipulation of polarizations of weak electromagnetic waves often times found in optical characterization4, sensing5, quantum computation6, 3D imaging7, etc. Relative to the linear polarization (LP) where the oscillation of its electric field lies along a single line, circular polarization (CP) whose field rotates either clockwise or counter clockwise is intrinsically more difficult to produce because of the required additional phase delay between the two orthogonally linearly polarized wave components. Conventional methods such as birefringence, all rely on phase retardation that is gradually accumulated as light propagates through an optical medium over a distance much longer than its wavelength. As such, significant thickness must be provided in order to obtain the needed phase retardation which dictates that nearly all optical components based on these conventional methods of manipulating polarizations are inevitably bulky. In addition, these bulky devices are always designed to perform only a single functionality, in the case of a polarizer, it yields light output in one particular polarizing state. Needless to say, a versatile, compact polarization generator capable of producing multiple polarizing states is highly desirable. Metamaterials have been extensively explored in recent years due to their exotic electromagnetic properties, such as negative refraction8, cloaking9, Fano resonance10, toroidal dipolar response11, and so on. A number of studies have demonstrated CP light 3


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 38

using complex three-dimensional (3D) chiral metamaterials such as multilayer gold spirals12,

13

. While these designs are capable of polarization modulation, there are a

number of disadvantages: 1) structures are optically thick as they still rely on light propagation in the media to accumulate phase; 2) light output is significantly attenuated at visible wavelengths because of the need for light to propagate through the plasmonic metamaterials with notoriously associated metal loss; and 3) the fabrication of the 3D structures is extremely complex. These factors fundamentally and technically limit their development and ultimately their scope of applications in optical systems, especially in integrated photonics, as efficient and compact polarizers. In contrast to 3D metamaterials, a metasurface capable of inducing abrupt modulation of the wavefront as it bounces off from it offers a promising alternative for manipulating polarization14. The phase modulation required for generating a particular polarization is typically obtained within a subwavelength skin depth of the metasurface in such a reflective scheme, making it possible to achieve flat, ultrathin polarization generators. So far a few flat optical components such as waveplates15, 16 and focusing lens17, 18 have been demonstrated with the use of metasurfaces patterned with noble metals such as gold (Au) and silver (Ag). The working wavelengths of these flat components are largely limited to spectral regimes longer than visible in order to avoid the substantial loss associated with the interband transitions in these metals in the visible range. It has been suggested that aluminum (Al) may be used as the metallic material of choice in a range of plasmonic devices and metasurfaces, expanding their operation into the visible and even UV regimes19, 20 where the photon energies are higher than interband transition energies in Al so that 4


Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

significantly reduced absorption can be achieved in comparison with Au and Ag, leading to the possibility of flat optics, color printing and hologram made of Al metasurfaces covering the whole spectral range of visible light21-23. In this work we numerically and experimentally demonstrate a metasurface polarization generator (MPG) that transforms a linearly polarized light into six states of polarization in visible with use of Al plasmonics. It should be noted that the functionality of the proposed MPG is obviously not limited to this particular demonstration. In fact, the salient feature of the MPG is its ability to generate multiple light beams of any polarizations from a single source of any specific polarization scattered from a single properly designed MPG device. In this demonstration, the polarizing state of the scattered light is fully controlled with the superposition between CPs (see Fig. 1a) of equal amplitude that undergoes 0-to-2π phase modulation by the metasurface. We have employed the Pancharatnam-Berry (P-B) phase24 (also referred to as geometric phase) method to realize the optimal control of the CP phase. Unlike the commonly used phase modulation methods in metasurfaces that rely on multi-resonances determined by metal patterns of varying sizes arranged along a particular orientation25, the P-B phase method achieves phase modulation through space-variant optical axis orientations of the plasmonic structures of equal dimensions26. Because of the space-invariant structural dimensions, such a technique offers the advantage of decoupling the phase modulation from the amplitude modulation for the scattered light at a fixed incident wavelength. In addition, the P-B phase method applied to Al plasmonic metasurfaces works independently at each wavelength over the wide visible and UV range, making it feasible 5


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 38

for broadband operation. Here we demonstrated a single MPG device with Al plasmonic structures that produces four LPs (horizontal, vertical, +45° and -45°) and two CPs (lefthand, LCP and right-hand, RCP) from a single linearly polarized source. Owing to the anomalous reflectance of the P-B phase method implemented with phase-modulating supercells of different lengths, all six light beams of different polarizations split into six directions, making them convenient to be used as subsequent light sources and easy to detect and analyze as they emerge background free at angles different from either the incident light source or normal reflection of the metasurface (Fig. 1a). The spatial offset between the nearby supercells of equal length is used to induce the needed phase delay between LCP and RCP that are reflected to the same anomalous angle so that their superposition can produce light wave of any desired polarization. Design of the metasurface polarization generator The basic building blocks of MPG are Al nanoantennas of equal size. When they are all arranged parallel along a fixed direction, an incident wave of LCP can be converted to that of RCP and vice versa. Figure 1b shows the optical characteristic of Al nanoantennas of equal dimensions of 170 nm × 50 nm × 50 nm patterned with the period of 230 nm × 230 nm along the same orientation on a SiO2 layer over a 150-nm thick Al mirror deposited on a Si substrate. The SiO2-layer thickness is optimized for high conversion efficiency

while

minimizing

co-reflectance

and

absorption27

of

the

Al

6


Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

nanoantenna/SiO2/Al film structure (see Supporting Information part 1). To achieve polarization conversion for circularly polarized light in a wide range of wavelength, the Al nanoantenna needs to exhibit two fundamental resonances that are separated far apart in wavelength, one along its long axis and the other short-axis, acting as a half-wave plate28 (see Supplementary part 2). According to our simulation, conversion efficiency for either LCP-to-RCP or RCP-to-LCP remains above ~50% over the visible region of 400 to 700 nm when the SiO2-layer thickness is chosen to be 50 nm. For longer wavelengths beyond 700 nm, interband transitions in Al begin to induce significant loss23, 29, 30 and cause conversion efficiency to drop. To extend its operation into the infrared, other metals such as Ag or Au can be considered with Ag holding some advantage over Au for being of lower loss but suffering from oxidization which can limit its functionality.

We then employed the P-B phase method by rotating the orientations of nanoantennas gradually with a constant step size of θ from 0 to π to create a supercell as shown in Fig. 1c. Each orientation angle change of θ will result in a 2θ phase change for the impinging CP light, resulting in 0-to-2π phase modulation. Because of the equal dimensions of these nanoantennas, the conversion efficiency mostly determined by the amplitude modulation remains unchanged with respect to the various locations in the supercell. As a result, metasurfaces made of these supercells have the ability to modulate phase without 7


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 38

significant relative amplitude modification at different positions, which forms the underlining design principle of our MPG device. To demonstrate this, we have simulated the phase modulation and conversion efficiency – a measure of amplitude modulation for eight periodical arrays, each made of Al nanoantennas of equal dimensions arranged in a particular angle θ, as illustrated in Fig. 1c. Indeed, 2π-phase modulation can be achieved and the conversion efficiency for each set of antennas at a fixed orientation is reasonably flat at about 73% at the wavelength of 500 nm. A subsequent construction of a supercell with 8 Al nanoantennas each orientated with the augmenting angle step of π/8 (the number of Al nanoantennas in a supercell is also referred to as number of levels for the phase modulation) should produce the needed 2π-phase modulation without the amplitude modulation. A MPG device can be designed and fabricated by arranging such supercells in various patterns (Fig. 1a) at different locations of the metasurface to achieve the necessary phase gradients for the desired polarization light beams to be scattered into pre-determined anomalous reflection angles. According to the generalized Snell’s law31, the angle of anomalous reflection θr from a MPG can be determined by the incident wavelength λ in free space and supercell length Lx, as θr = sin-1(λ /Lx) under a normal illumination. For example, when the length of supercell is fixed (i.e., the number of Al nanoantennas in a supercell is fixed since each nanoantenna occupies a fixed unit area of 230 nm × 230 nm in our design), the angle of anomalous reflection becomes larger when the incident wavelength is longer, as shown by simulation in Fig. 2a. The result also indicates the P-B phase-based MPG made of Al nanoantenna supercells works well across the visible spectral range. The simulated 8


Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

conversion efficiency has been plotted as a function of wavelength in Fig. 2c and can be found to reach maximum at λ = 500 nm, consistent with the prediction from the simulation result in Fig. 1b. The number of levels (or the number of Al nanoantennas in a supercell) employed in the phase gradient also plays a role in the angle of anomalous reflection because higher level numbers require greater supercell length to implement since each Al nanoantenna occupies a fixed unit area as shown in Fig. 2b. We have found through simulation that the maximum CP conversion efficiency increases with the level number of phase gradient as shown in Fig. 2d. It is not difficult to see that the higher level number offers a smoother phase gradient across the metasurface, resulting in higher conversion efficiency32. But the conversion efficiency tends to reach a plateau when the level number surpasses 8. We therefore choose to keep the minimum level number of phase gradient at 8 as we design the MPG. We now discuss the design principle of the MPG using the above simulation results obtained from the Al metasurface. Considering that the MPG reflects a LCP light beam at normal incidence to a RCP light with its phase gradient at an anomalous angle +θr, then a RCP at normal incidence will result in a LCP reflection at -θr. By reversing the order of the supercells arranged on the metasurface, we arrive at inverse optical response from the original polarization26. As a result, a LP light which can be viewed as a combination of LCP and RCP, upon impinging upon the metasurface, will create two CP reflection

9


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 38

beams with opposite handedness propagating in two different directions, ±θr, simultaneously, as shown in Fig. 3a. Once LCP and RCP are produced, we can use them to generate LP states by simply superpositioning the orthogonal LCP and RCP through combining two supercells with opposite nanoantenna orientations. The MPG consists of two supercells with a position offset d, providing a phase difference between the two CPs needed to produce LP scattered wave along a particular direction33 (see Fig. 3b). Basically, the RCP ( R ) and LCP ( L ) waves can be respectively described in terms of Jones vectors as: R =

1 1   2  −i 

(1)

1 1 L =  2 i 

The position offset d between two supercells is implemented to yield a phase delay α = k0dsin(θr) = 2πd / Lx between the two scattered LCP and RCP. Here, k0 = 2π/λ is the wavenumber in free space. The superposition of these two CP waves can then be given as: n = R + eiα ( d , Lx ) L

(2)

where n represents a LP wave when the position offset d is chosen properly with a fixed length of supercell Lx. For demonstration purpose, we have put together two supercells with different position offsets to generate six kinds of polarizations all from a 10


Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

single MPG device, including LCP, RCP, LP along horizontal (LP-H) corresponding to x polarization, LP along vertical (LP-V) corresponding to y polarization, LP along +45° (LP+45˚), and RP along -45° (LP-45˚). Since the angle of anomalous reflection is related to the length of supercell which is determined by the level number of phase gradient at a fixed wavelength, we can separate the different polarization beams into different reflection angles by adopting three phase-gradient level numbers: 8 for LP+45˚ and LP-45 ˚, 10 for LP-H and LP-V, and 12 for LCP and RCP, making these different polarization beams easily separated from one another as well as from incident and normal reflection waves. Figure 3c shows the numerical simulation results for the conversion efficiency of the scattering light of different polarizations for three structural configurations. The anomalous reflection angle of scattered light with designed polarizing state agrees almost exactly with the generalized Snell’s law. The six polarizing states can be observed by detecting their electric field components generated by these three structural configurations, as shown in Fig. 3c. It is worth mentioning that the level number of phase gradient can be chosen at will and one can obtain a given polarization state at any desired observation angle. Experimental characterization of metasurface polarization generator The Stokes vector described below can be conveniently used to evaluate any polarization states:

11


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2  E  S0   S   E 2 − E 2 LHP LVP 1 S= = 2  S2   E − EL − 45 P    L + 45 P 2 2  S3   E  RCP − ELCP

Page 12 of 38

   2   

(3)

where the Stokes parameters Si (i = 0, 1, 2 and 3) are a set of values related to the amplitudes of polarizing states. Figure 4a shows the optical setup to measure the Stokes parameters and characterize the generated polarizing states of the fabricated MPG under a LP-H wave illumination at normal incidence. A supercontinuum laser combined with an acousto-optic filter was employed to generate a series of discrete, equally spaced laser peaks at a desired incident wavelength. Polarizer (P1) is used to generate a LP-H beam from the incident laser source. Upon impinging on the MPG, the scattered light was projected onto the focal plane of the lens (L). Light diffracted by the MPG then goes through a broadband quarter-wave plate (λ/4) and another polarizer (P2), and is then collected by a sCMOS camera. The distance between the MPG and sCMOS camera is optimized for detecting simultaneously all six scattered light spots with different polarizations (see Supplementary Part 4). We subsequently obtain a plot of signal intensity as a function of reflection angle of fast axis φ of quarter-wave plate. The intensity variation can be calculated by Mueller matrix34: 12


Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1 I (ϕ ) = [ A − B sin(2ϕ ) + C cos(4ϕ ) + D sin(4ϕ )] 2

(4)

where

A= B= C= D=

1

π 1

π 1

π 1

π

2π

∫ I (ϕ ) dϕ 0

2π

∫ I (ϕ ) sin(2ϕ ) dϕ 0

2π

(5)

∫ I (ϕ ) cos(4ϕ ) dϕ 0

2π

∫ I (ϕ ) sin(4ϕ ) dϕ 0

These parameters are related to Stokes parameters as S0 = A – C, S1 = 2C, S2 = 2D, S3 = B (see Supplementary Part 5) and therefore all Stokes parameters of scattered light from the designed MPG can be obtained via the scattered intensity with respect to φ. We have fabricated a MPG device consisting of six areas, each made of one type of supercells responsible for generating a light beam of a particular polarizing state into a pre-determined scattering angle. The six different areas fabricated using electron beam lithography on a single metasurface chip are shown in the center column of Fig. 4b along with their theoretical and experimental results for the LP-H incident wave at λ = 600 nm where lines of different color represent different polarizations and correspond to their SEM images of the same labeled color. The theoretical and measured results show a good 13


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

agreement. The slight difference can be attributed to the imperfect shapes of the Al nanoantennas, surface oxidization of Al nanoantennas, and alignment issue in the optical measurement as well as non-ideal optical components (especially the quarter-wave plate). These results can be plotted on the Poincaré sphere for evaluation of generated polarizing states. Using the Stokes parameters as the sphere’s coordinates, the Poincaré sphere offers a prominent three-dimensional space geometric representation of polarization. Any polarizing state can be uniquely described as a point on or within a unit sphere. It is a convenient presentation that makes polarization transformation clear to view. Figure 4c shows the comparison of experimental results (color dots) and theoretical predictions (black dots) at three different wavelengths, red (650 nm), green (550 nm), and blue (450 nm). Again, the measured results show a good agreement with the theoretical prediction. More experimental results are presented in Supplementary, validating that the proposed MPG is indeed capable of operating in the broad visible spectrum.

Discussion

14


Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

The performance of the MPG is further analyzed by measuring the extinction ratio of a given generated polarization. This is done by examining the ratio of optical power of the desired polarization to that of its orthogonal counterpart. The experimental results indicate that all generated polarizing states delivered high extinction ratios over the wide range of visible spectrum, as shown in Fig. 5. These extinction ratios are significantly higher than those of previously reported from either metamaterials or metasurfaces and the purity of generated polarization is on a par with commercial products. In addition to the high polarization extinction ratio, the anomalous reflection beam of a given polarizing state allowed for its background-free detection by conveniently avoiding the experimental setup complication arising from the incident and normal reflective signals during the measurement. We could have easily designed the MPG to produce elliptically polarized wave in addition to the LP and CP waves by simply tuning the position offset d. Such MPG can also work as a polarization-dependent beam splitter to decompose any incident light into two orthogonal polarizations and separate them into different angles.

Conclusions We have demonstrated a MPG capable of generating multiple polarization waves with high polarization extinction ratio in a reflective scheme. Using Al nanoantennas of equal 15


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

dimensions as the basic building blocks and achieving phase modulation through tuning of their optical axis orientations, we have designed and fabricated the MPG consisting of six areas, each with a patterned array of supercells made of Al nanoantennas arranged with an incremental step of orientating angle to realize the 2π-phase modulation without significant relative amplitude modification at different locations of the metasurface. We have implemented in a supercell with the level number of phase gradient no less than 8 to yield six different polarization beams all with high polarization conversion efficiency and demonstrated that the Al plasmonic MPG can indeed operate in the wide visible spectrum. The reflective MPG device fabricated on top of a SiO2/Al/Si ground plate has produced much higher conversion efficiency than that of transmission type previously reported by other groups. To further increase the conversion efficiency, it is desirable to replace any metal on the MPG with patterned low-loss dielectric features made of materials such as TiO2, GaN, etc. as the active components that interact with incident and scattered light for polarization generation. The purity of polarization is the highest among all reported with either metamaterials or metasurfaces and on par with commercial products. The low cost and ease of adaptability to various platforms make Al plasmonic MPGs particularly promising with low economic threshold for mass production and thus can be made widely 16


Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

available for a broad range of applications. In comparison with commercial polarization generators/modulators that are available in the market, our MPG device is optical thin, easy to fabricate, and compatible with flat optics. With the validation of generating multiple waves of different polarizations from a single metasurface, we demonstrate that MPG made of Al is an effective and versatile optical component that can be integrated in photonic circuits and potentially expands the reach of nanotechnology.

Methods Fabrication of MPG. All samples are prepared by standard electron beam lithography and lift-off process. A 150-nm-thick aluminum mirror and 50-nm-thick SiO2 were first deposited by magnetron sputtering machine (Shibaura Mechatronics Corp.) on a silicon substrate. The 495K PMMA (polymethyl methacrylate) photoresist was then deposited by spin coating and baked at 180 °C for 3 minutes. Before the e-beam exposure, a conductive polymer layer Espacer, which can be directly removed by rinsing with deionized water is spin-coated over the PMMA layer to eliminate the charging issue. After exposure (with 100 keV acceleration voltage and 30 pA current) the sample was rinsed with de-ionized water to remove Espacer, then developed in solution of methyl 17


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

isobutyl ketone (MIBK) and isopropyl alcohol (IPA) of MIBK : IPA = 1:3 for 60 seconds, subsequently rinsed with IPA for 20 seconds, and blow-dried with nitrogen gas. Aluminum with desired thickness was then deposited via e-beam evaporator. The aluminum nanoantennas were consequently deﬁned on the substrate after the lift-off process.

Simulation. All simulation results were performed with the commercial software CST Microwave Studio. For the case of single cell design, a unit cell boundary condition is employed for the simulation of reflection and phase shift in an array structure. The refractive index of SiO2 is obtained from Ref. 35. The permittivity of aluminum in the visible regime is described by the Drude model.

Acknowledgments The authors acknowledge financial support from Ministry of Science and Technology, Taiwan (Grant No. MOST-105-2745-M-002-002-ASP) and Academia Sinica (Grant No. AS-103-TP-A06). They are also grateful to National Center for Theoretical Sciences, NEMS Research Center of National Taiwan University, National Center for HighPerformance Computing, Taiwan, and Research Center for Applied Sciences, Academia

18


Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Sinica, Taiwan for their supports. G. Sun acknowledges support from AFOSR (FA955014-1-0196) and AOARD (FA2386-16-1-4069).

ASSOCIATED CONTENT Supporting Information Available: Detailed information on optical responses for aluminum-nanoantenna/SiO2/aluminum-mirror structure, fundamental resonances and half-wave plate behavior of Al nanoantenna, camera images of generated polarizing states, description of Stokes parameters via Mueller matrix and additional experimental results. This material is available free of charge via the internet at http://pubs.acs.org.

Author Contributions P. C. W. and W. Y. T. contributed equally to this work.

Notes The authors declare no competing financial interests.

19


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

References 1. 2.

Bahadur, B. Mol. Cryst. Liq. Cryst. 1984, 109, 3-93. Calabretta, N.; Liu, Y.; Huijskens, F. M.; Hill, M. T.; Waardt, H. d.; Khoe, G. D.; Dorren, H. J. S. J. Lightwave Technol. 2004, 22, 372-381.

3.

Young, L.; Robinson, L.; Hacking, C. IEEE Trans. Antenn. Propag. 1973, 21, 376-378.

4.

Mühlschlegel, P.; Eisler, H. J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W. Science 2005, 308, 16071609.

5.

Hariharan, P. C.; Pople, J. A. Theoretica chimica acta 1973, 28, 213-222.

6.

Walther, P.; Resch, K. J.; Rudolph, T.; Schenck, E.; Weinfurter, H.; Vedral, V.; Aspelmeyer, M.; Zeilinger, A. Nature 2005, 434, 169-176.

7.

Dickson, R. M.; Norris, D. J.; Tzeng, Y.-L.; Moerner, W. E. Science 1996, 274, 966-968.

8.

Smith, D. R.; Pendry, J. B.; Wiltshire, M. C. K. Science 2004, 305, 788-792.

9.

Gharghi, M.; Gladden, C.; Zentgraf, T.; Liu, Y.; Yin, X.; Valentine, J.; Zhang, X. Nano Lett. 2011, 11, 2825-2828.

10.

Wu, P. C.; Chen, W. T.; Yang, K.-Y.; Hsiao, C. T.; Sun, G.; Liu, A. Q.; Zheludev, N.; Tsai, D. P. Nanophotonics 2012, 1, 131-138.

11.

Huang, Y.-W.; Chen, W. T.; Wu, P. C.; Fedotov, V. A.; Zheludev, N. I.; Tsai, D. P. Sci. Rep. 2013, 3, 1237.

12.

Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M. Science 2009, 325, 1513-1515.

13.

Zhao, Y.; Belkin, M. A.; Alù, A. Nat. Commun. 2012, 3, 870.

14.

Minovich, A. E.; Miroshnichenko, A. E.; Bykov, A. Y.; Murzina, T. V.; Neshev, D. N.; Kivshar, Y. S. Laser Photon. Rev. 2015, 9, 195-213.

15.

Ding, F.; Wang, Z.; He, S.; Shalaev, V. M.; Kildishev, A. V. ACS Nano 2015, 9, 4111-4119.

16.

Zhao, Y.; Alù, A. Nano Lett. 2013, 13, 1086-1091.

17.

Aieta, F.; Genevet, P.; Kats, M. A.; Yu, N.; Blanchard, R.; Gaburro, Z.; Capasso, F. Nano Lett. 2012, 12, 4932-4936.

18.

Chen, X.; Huang, L.; Mühlenbernd, H.; Li, G.; Bai, B.; Tan, Q.; Jin, G.; Qiu, C.-W.; Zhang, S.; Zentgraf, T. Nat. Commun. 2012, 3, 1198.

19.

James, T. D.; Mulvaney, P.; Roberts, A. Nano Lett. 2016, 16, 3817-3823.

20.

Olson, J.; Manjavacas, A.; Liu, L.; Chang, W.-S.; Foerster, B.; King, N. S.; Knight, M. W.; Nordlander, P.; Halas, N. J.; Link, S. Proc. Natl. Acad. Sci. 2014, 111, 14348-14353.

21.

Yokogawa, S.; Burgos, S. P.; Atwater, H. A. Nano Lett. 2012, 12, 4349-4354. 20


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

22.

Miyata, M.; Hatada, H.; Takahara, J. Nano Lett. 2016, 16, 3166-3172.

23.

Huang, Y.-W.; Chen, W. T.; Tsai, W.-Y.; Wu, P. C.; Wang, C.-M.; Sun, G.; Tsai, D. P. Nano Lett. 2015, 15, 3122-3127.

24.

Ding, X.; Monticone, F.; Zhang, K.; Zhang, L.; Gao, D.; Burokur, S. N.; de Lustrac, A.; Wu, Q.; Qiu, C.-W.; Alù, A. Adv. Mater. 2015, 27, 1195-1200.

25.

Yu, N.; Capasso, F. Nat. Mater. 2014, 13, 139-150.

26.

Huang, L.; Chen, X.; Mühlenbernd, H.; Li, G.; Bai, B.; Tan, Q.; Jin, G.; Zentgraf, T.; Zhang, S. Nano Lett. 2012, 12, 5750-5755.

27.

Landy, N. I.; Sajuyigbe, S.; Mock, J. J.; Smith, D. R.; Padilla, W. J. Phys. Rev. Lett. 2008, 100, 207402.

28.

Khorasaninejad, M.; Crozier, K. B. Nat. Commun. 2014, 5, 5386.

29.

Knight, M. W.; King, N. S.; Liu, L.; Everitt, H. O.; Nordlander, P.; Halas, N. J. ACS Nano 2013, 8, 834-840.

30.

Sobhani, A.; Manjavacas, A.; Cao, Y.; McClain, M. J.; García de Abajo, F. J.; Nordlander, P.; Halas, N. J. Nano Lett. 2015, 15, 6946-6951.

31.

Yu, N.; Genevet, P.; Kats, M. A.; Aieta, F.; Tetienne, J.-P.; Capasso, F.; Gaburro, Z. Science 2011, 334, 333-337.

32.

Zheng, G.; Mühlenbernd, H.; Kenney, M.; Li, G.; Zentgraf, T.; Zhang, S. Nat. Nano. 2015, 10, 308312.

33.

Yu, N.; Aieta, F.; Genevet, P.; Kats, M. A.; Gaburro, Z.; Capasso, F. Nano Lett. 2012, 12, 63286333.

34.

Goldstein, D. Polarized Light. CRC Press; 3 edition, 2016.

35.

Ghosh, G. Opt. Commun. 1999, 163, 95-102.

21


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

FIGURES AND FIGURE CAPTIONS

Figure 1. Polarization conversion with aluminum plasmonic metasurfaces. (a) Schematic for arbitrary polarization generation with fixed incident polarization. (b) Simulation and measurement results for the LCP-to-RCP conversion efficiency by an Al nanoantenna array deposited on SiO2/Al/Si substrate over the spectral range of 400 nm to 1000 nm. The simulation is conducted on the nanoantenna array with the unitcell dimensions of length L = 170 nm, width S = 50 nm, thickness H1 = 50 nm, and period P = 230 nm that are deposited on SiO2 layer with thickness H2 = 50 nm and Al mirror thickness H3 = 150 nm. The measurement is performed on a sample whose scanning electron microscope (SEM) image is shown in inset. (c) Conversion efficiency and phase modulation for Al nanoantennas of equal dimensions oriented at various angles at wavelength λ = 500 nm. Each data point represents an array of Al nanoantennas all oriented at a fix angle θ.

22


Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. Simulated conversion efficiency and angle of reflection associated with incident wavelength and level number of phase gradient. Conversion efficiency and angle of reflection by (a) a MPG with a fixed supercell length (Lx = 230 nm × 8 = 1840 nm) for a range of incident wavelengths; (b) a MPG of different lengths designed with different level numbers of phase gradient under the illumination of a single wavelength (λ = 500 nm). Peak conversion efficiency as a function of (c) incident wavelength with the level number of phase gradient fixed at 8 and (d) level number at the fixed incident wavelength of (λ = 500 nm) obtained from (a) and (b), respectively. All the incident waves are polarized along the horizontal x-axis.

23


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

Figure 3. Polarization generation with superposition of CPs. Schematics for (a) circular polarization generation through intrinsic property of P-B phase-based metasurfaces and (b) linear polarization generation through superposition of CPs under an x-polarized (LP-H) light illumination. (c) Simulated conversion efficiency for each polarization at the incident wavelength of 500 nm. All insets present the schematic of corresponding supercell design. The lengths of supercells Lx for RCP and LCP, LP-H and LPV and LP+45˚ and LP-45˚ are 2.76 µm, 2.3 µm and 1.84 µm, respectively.

24


Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. Experimental verification of generated polarizations. (a) Measurement setup for the MPG characterization. (b) Theoretical (left column)) and experimental (right column) diagram of scattering intensity as a function of rotating angle of the quarter-wave plate. Here, the LP-H incident wavelength λ = 600 nm. Middle column: corresponding SEM images with 1-µm scale bar. Six lines display the results for six polarizations. (c) Poincaré sphere experimental results (red/green/blue dots) compared with theoretical predictions (black dots) for different incident wavelengths.

25


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

Figure 5. Polarization extinction ratio. Measured polarization extinction ratio of the six generated polarizing states over a wide range of visible light.

26


Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Table of Contents graphic

27


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

387x283mm (96 x 96 DPI)


Page 28 of 38

Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

396x232mm (96 x 96 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

370x204mm (96 x 96 DPI)


Page 30 of 38

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

329x299mm (96 x 96 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

235x184mm (96 x 96 DPI)


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

539x170mm (96 x 96 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

378x194mm (96 x 96 DPI)


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

414x206mm (96 x 96 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

451x182mm (96 x 96 DPI)


Page 36 of 38

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

328x100mm (96 x 96 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents graphic 264x198mm (96 x 96 DPI)


Page 38 of 38

Versatile Polarization Generation with an Aluminum Plasmonic

Recommend Documents